It is known to use X-rays or beams of neutrons to effect various analyses of materials.
A source of X-rays or neutrons is necessary for this, and a monochromator device is generally used. The purpose of the monochromator device is to select a wider or narrower band of wavelengths (i.e. energy) from the spectrum of the source whose extent in terms of wavelength is too large for the envisaged application.
For X-rays, the selection of a band of wavelengths is effected by means of the phenomenon of diffraction of X-rays by a perfect crystal.
Accordingly, incident X-rays whose spectrum extends over a given range of wavelengths received by a perfect crystal at a given angle of incidence give rise to diffraction of the radiation in a narrower band of wavelengths.
It will be noted that the width of the band of wavelengths diffracted by the crystal depends on the nature of the crystal used (lattice parameter, symmetry of the crystal) and on the crystallographic line chosen.
It is in particular known to use silicon as the perfect crystal, being a material well-known for the quality and the sufficient size of its crystals, for the ease with which it can be worked, and for its low cost.
However, the bandwidth of silicon proves to be too small compared to the bandwidth of the sources used and this leads to a considerable loss of flux. For example, for a source of X-rays used in the laboratory (for example employing a cathode ray tube or a rotary anode), from an emission line of a metal such as copper or molybdenum, the width of a fluorescence line is conventionally of the order of ΔE/E=3-5×10−4, whereas the bandwidth of silicon 111 is 1.3×10−4, which means that two thirds of the intensity of the incident radiation are lost. Silicon has too high a resolution for applications using the X-ray diffraction technique.
It is also known to use germanium as the perfect crystal, being a material available in the form of large perfect crystals and which, because of a higher electron density than silicon, and thus greater line widths, transmits three times the flux transmitted by a silicon crystal.
For example, the line width of 111 germanium (Δλ/λ=3×10−4) is well adapted to the case of a source formed of fluorescence lines the width of which is of the order of 3-5×10−4(see above).
However, the cost of a material such as germanium is higher than that of silicon and its mechanical characteristics (in particular its elastic limit) and its thermal characteristics (in particular its thermal conductivity) are worse than those of silicon. Because of this, with germanium as the crystal, it is difficult to envisage applications in which the curvature of the crystal must be variable and change as a function of the application. Such applications are encountered when it is required, for example, to focus X-rays at variable distances to adapt the optics to the apparatus or to focus different energies at a fixed distance.
The object of this focusing is to reduce the size of the beam produced at the location of the sample to be analyzed.